STANBOL-1023: The Sling OSGI Installer plugin for installing SolrCores on ManagedSolrServers not register itself only after the first ManagedSolrServer is available. By this the timing issues mentioned in the issues are avoided

git-svn-id: https://svn.apache.org/repos/asf/stanbol/trunk@1467763 13f79535-47bb-0310-9956-ffa450edef68